As an important mechanism of the system of long term evolution (LTE), the random access procedure impacts user experience. Specifically, random access procedure is the access procedure before a user equipment (UE) communicates with network at begins; it can be understood as a channel application process. The random access procedure is performed for the following six events: initial access from RRC_IDLE, radio resource control (RRC) connection re-establishment procedure, handover, downlink (DL) & uplink (UL) data arrival during RRC_CONNECTED requiring random access procedure, and for positioning purpose during RRC_CONNECTED requiring random access procedure. For the events of DL data arrival and handover, non-contention based random access procedure could be used, as shown in FIG. 1, comprising the following steps:
Step 101, the base station assigns to UE a non-contention Random Access Preamble via dedicated signaling in DL.
Specifically, the signaling includes handover (HO) command generated by target eNB and sent via source eNB for handover and HO command generated by target eNB and sent via source eNB for handover;
Step 102, UE transmits the assigned non-contention Random Access Preamble.
Step 103, the base station calculates the uplink timing advance (TA) based on the preamble reception.
Step 104, the base station sends the random access response (RAR), and UE derives the TA information included in the RAR to obtain the uplink synchronization.
For the other events, contention based random access procedure would be used, as shown in FIG. 2, comprising the following steps:
Step 201, UE selects the random access preamble and packet random access channel (PRACH) resource which is obtained from system information and transmits the preamble on RACH to the base station.
Step 202, the base station calculates TA based on the random access preamble reception, and sends random access response to UE. The RAR message Conveys at least RA-preamble identifier, TA information, initial UL grant and assignment of Temporary C-RNTI
Step 203, UE performs the uplink transmission in the assigned UL grant.
Specifically, to different random access reasons, the content of uplink transmission is different. For example, to initial access, the content of uplink transmission is RRC connection request.
Step 204, UE receives the contention resolution message from the base station, and judges whether the random access is successful.
Besides the above random access mechanism, LTE system also designs uplink synchronous procedure, which is used for keeping the uplink synchronization between UE and base station, in order for UE to transmit uplink data and hybrid automatic repeat quest (HARQ) feedback which is related to the DL transmission. In LTE system base station is in charge of uplink synchronization maintenance. Specifically, when UE performing random access, the base station acquires original TA by preamble, the following uplink synchronous maintenance is as shown in FIG. 3, comprising: the base station and UE maintain a timing advance timer (TAT) respectively, the base station sends TA command to UE, if the UE can't receive TA command correctly, then the UE does nothing to TAT, and the base station resends TA command; if the UE receives TA command correctly, then UE starts the TAT IN UE side and sends HARQ-ACK information to the base station, the base station restarts its TAT based on it. If the TAT is running, the base station and UE would regards the uplink sync state is in-sync.
As a evolution of LTE technology, the peak rate of LTE-A system has been greatly improved relative to the LTE system, which can be reached as downlink 1 Gbps and uplink 500 Mbps. Of course, LTE-A system need to keep good backward-compatibility with the LTE system. Based on the demand of peak rate improvement, backward-compatibility with LTE system and improving the spectrum efficiency, the LTE-A system introduces carrier aggregation (CA) technology. The CA technology is that the uplink and downlink includes more than one component carrier (CC) respectively, in the LTE system UE only works in one carrier. In the system with carrier aggregation, the aggregated carriers can be continuous or non-continuous, to compatible with LTE system, the max bandwidth of each carrier is 20 MHz, the bandwidth of each carrier can be same or different.
There are four deployment scenarios of uplink synchronization in 3rd generation partnership project radio access network (3GPP RAN4) to: for the scenarios without repeater, radio remote unit (RRU) and coordinated multiple points (CoMP), TA of each CC is the same, as shown in FIG. 4; for the scenarios with repeater, since repeater has the different frequency selectivity, TA of each CC may be different, as shown in FIG. 5; In Non-collocated Sites, TA of each CC may be different, as shown in FIG. 6; To uplink CoMP, the TA is different to the different sites of participating in collaboration.
In the process of achieving this present invention, the inventor found that there are at least the problems of existing technology:
In LTE-A system, with the introduction of multicarrier, the uplink synchronization timing advance of each CC may be different. In the present LTE-A agreement, there is no any description and design about the uplink synchronization establishing and maintaining the mechanism.